1. Field of the Invention
The present invention relates to a hydraulic pressure control device for a drive torque distribution clutch, such as a two wheel drive/four wheel drive switchover clutch or a center differential lockup clutch, which controls drive torque distribution in a four wheel drive device in a vehicle such as an automobile.
2. Description of the Prior Art
The four wheel drive device mounted in a vehicle such as an automobile is known in the form of the so-called part-time type and the so-called full-time type, and in either type it incorporates a clutch for controlling drive torque distribution in the four wheel drive device. In the part-time type four wheel drive device, either a front wheel drive member or a rear wheel drive member is directly connected with a drive member adapted to be driven by an engine, and a switchover clutch selectively couples the other of the front wheel drive member and the rear wheel drive member to the drive member driven by the engine, so as thereby to effect switching over between the two wheel drive state and the four wheel drive state. The full-time type four wheel drive device has a center differential device for differentiatingly driving a front wheel drive member and a rear wheel drive member from an engine, and a center differential lock-up clutch which selectively locks up the differential operation of the center differentiating device so as thereby to effect switching over between a front/rear differential drive state and a positive four wheel drive state.
Such a switchover clutch or central differential lock-up clutch as described above is generally not to operate only in a completely engaged state or a completely disengaged state but continually controls the amount of drive torque transmitted therethrough so that continually variable intermediate drive states are available between the four wheel drive state and the two wheel drive state or between the complete four wheel differential drive state and the four wheel positive drive state.
Various proposals have already been made to control the torque transmission capacity or the engagement state of such a clutch according to various control parameters. Among such prior proposals are Japanese Patent Laying-open Publication 60-176827 (1985), Japanese Utility Model Laying-open Publication 61-101034 (1986), and Japanese Patent Laying-open Publication 62-139772 (1987).
In such four wheel drive devices the clutch for controlling torque distribution is generally of a hydraulic control type in which the transmission torque is variably set according to the hydraulic pressure supplied thereto, and the torque transmission capacity of the torque distribution control clutch is controlled by a control of the duty ratio of a hydraulic pressure supplied to said clutch by employing an electric control means.
Based upon such a duty ratio control the hydraulic pressure will be controlled continuously, and the torque transmission capacity of the torque distribution control clutch will be continuously controlled. Therefore, such a control system will be able to provide fine control.
However, such a duty ratio control is relatively slow in its response to an abrupt change in the hydraulic pressure required, and therefore it is difficult to obtain a quick switching over of the four wheel drive device between the four wheel drive state and the two wheel drive state or between the four wheel differentiating drive state and the four wheel positive drive state.
Further, when the vehicle is driven for a long time in the four wheel positive drive state, there is a danger that the engine is overloaded by the tight corner braking at curved running and the durability of the drive torque distribution clutch is thereby damaged. Therefore, it is desirable that the drive torque distribution clutch in the four wheel drive device is loosened or completely released when any trouble has occurred in the control system for the clutch.